1. Field of the Invention
This invention relates to a method for formation of a thin film multi-layer structure member for electronic devices such as thin film semiconductor devices, photovoltaic devices, photosensitive devices for electrophotography.
2. Related Background Art
In the prior art, for functional films, especially amorphous or polycrystalline semiconductor films individually suitable film forming methods have been employed from the standpoint of desired physical characteristics, uses, etc.
For example, for formation of silicon deposited films such as amorphous or polycrystalline non-single crystalline silicon which are optionally compensated for lone pair electrons with a compensating agent such as hydrogen atoms (H) or halogen atoms (X), etc., (herein-after abbreviated as "NON-Si (H,X)", particularly "A-Si (H,X)" when indicating an amorphous silicon and "poly-Si (H,X)" when indicating a polycrystalline silicon) (the so-called microcrystalline silicon is included within the category of A-Si (H,X) as a matter of course), there have been attempted the vacuum vapor deposition method, the plasma CVD method, the thermal CVD method, the reactive sputtering method, the ion plating method, the optical CVD method, etc. Generally, the plasma CVD method has been widely used and industrialized.
However, the reaction process in formation of a silicon-based deposited film according to the plasma CVD method which has been generalized in the prior art is considerably complicated as compared with the other CVD methods of the prior art, and its reaction mechanism involves not a few ambiguous points. Also, there are a large number of parameters for formation of a deposited film (for example, substrate temperature, flow rate and flow rate ratio of the introduced gases, pressure during formation, high frequency power, electrode structure, structure of the reaction vessel, speed of evacuation, plasma generating system, etc.). By combination of such a large number of parameters, the plasma may sometimes become unstable state, whereby marked deleterious influences were exerted frequently on the deposited film formed. Besides, the parameters characteristics of the device must be selected for each device and therefore under the present situation it has been difficult to generalize the production conditions.
On the other hand, for the silicon-based deposited film to exhibit sufficiently satisfactory electric and optical characteristics for respective uses, it is now accepted that the best to form it is according to the plasma CVD method.
However, depending on the application use of the silicon-based deposited film, bulk production with reproducibility must be attempted with full satisfaction of enlargement of area, uniformity of film thickness as well as uniformity of film quality, and therefore in formation of a silicon-based deposited film according to the plasma CVD method, enormous installation investment is required for a bulk production device and also management items for such bulk production become complicated, with management tolerance being narrow and the control of the device being severe. These are pointed as the problems to be improved in the future.
Also, in the case of the plasma CVD method, since plasma is directly generated by high frequency or microwave, etc., in the deposition space in which a substrate on which film is formed in arranged, electrons or a number of ion species generated may cause damage to the film in the film forming process to cause lowering in film quality or non-uniformization of film quality.
Particularly, in the case of a semiconductor device having a multi-layer structure, the state of the interfaces between the respective layers has been known to affect greatly the characteristics of the device. Accordingly, for example, in the case of preparing a photosensitive member for electrophotography, since starting gas species, flow rates and plasma discharging intensities, etc. differ largely from layer to layer during deposition of longer wavelength-light absorbing layer, charge injection preventing layer, photosensitive layer, surface protective layer, etc. on a substrate, complete gas exchange is effected with discharging being stopped, a varied layer is provided which is formed by varying gradually the gas species, flow rate and plasma discharging intensity, or alternatively the respective deposited films are formed in separated deposition chambers, thereby attempting to improve the device characteristics through improvement of the state of the interfaces between the respective deposited films. However, according to any of these methods, no satisfactory change or improvement of the device characteristics could be recognized.
As described above, in formation of silicon-based deposited film, the points to be solved still remain, and it has been earnestly desired to develop a method for forming a deposited film which is capable of bulk production by attempting to effect conservation of energy by means of a device of low cost, while maintaining the characteristics as well as uniformity which are practically available.
Particularly, it has been desired earnestly to develop a formation method which can improve the interface state of a thin film multi-layer structure member such as thin film transistors, photovoltaic devices, photosensitive members for electrophotography, etc. and a method for forming a multi-layer structure member having good interface state which can improve the characteristics of the devices.